Imagesetters and platesetters are used to expose substrates, which are used in many conventional offset printing systems. Imagesetters are typically used to expose the film that is then used to make the plates for the printing system. Platesetters are used to directly expose the plates.
For example, plates are typically large substrates that have been coated with photosensitive or thermally-sensitive material layers, referred to the emulsion. For large run applications, the substrates are fabricated from aluminum, although organic substrates, such as polyester or paper, are also available for smaller runs.
Computer-to-plate printing systems are used to render digitally stored print content onto these printing plates. Typically, a computer system is used to drive an imaging engine of the platesetter. In a common implementation, the plate is fixed to the outside or inside of a drum and then scanned with a modulated laser source in a raster fashion.
The imaging engine selectively exposes the emulsion that is coated on the plates. After this exposure, the emulsion is developed so that, during the printing process, inks will selectively adhere to the plate""s surface to transfer the ink to the print medium.
The imaging engines of these platesetters and/or imagesetters have imaging devices that generate powerful spatially and/or temporally modulated optical signals. These optical signals are used to expose the plate or film media held on the drum or flat bed. Typically, the media is held on a drum that is rotated underneath the imaging engine while the imaging engine is scanned axially along the drum to expose the media on the drum.
The process of exposing plates is basically a binary process. Where the plates are exposed, the ink either adheres or does not adhere, depending one the emulsion chemistry. As a result, grey images are generated by a halftoning process, in which dots of different sizes and/or densities are placed in order to get the desired color density when integrated by the human eye.
In negative-working plates, the exposure of the emulsion produces a dot through a photopolymerization process. Depending on whether the polymer is hydrophilic or hydrophobic, the ink then adheres to the dot or not in the subsequent offset printing process.
In contrast, in positive-working systems, the exposure of the emulsion causes the emulsion to be removed during the development process, leaving the exposed plate substrate, which is typically aluminum. Depending on the system, this exposed region of aluminum will either hold ink or not.
Even though the basic system is binary, exposure setting is critical. Exposure is set by the imaging engine of the platesetter or imagesetter. It is a function of the strength of the exposure beam and how long the exposure beam is resident at each pixel.
For example, under-exposure in a negative-working system will result in the generation of polymer dots that are insufficiently hard. As a result, they will not survive long in the offset printing machine, requiring the plates to be replaced frequently. On the other hand, over exposure will yield a dot that is too large, which will impact the balance of the color and non-colored areas in the final image.
The conventional process for setting exposure in the imaging engine involves the use of an analog wedge test target. This is attached to the plate, typically in a darkroom. The plate is then loaded into the imaging engine and exposed. Thereafter, the target is removed and the plate is developed. From analysis of the portion of the plate that was exposed through the step wedge, it is possible to determine the proper exposure setting for the imaging engine, as some fraction of the exposure beam""s power during the exposure through the analog step wedge.
A number of problems exist with this conventional system. First, the attachment to the step wedge to the plate is a time consuming process that requires operator intervention. To address this, some have suggested step wedges that are flipped into the path of the exposure beam, between the source and the plate. This xe2x80x9cmechanical solutionxe2x80x9d avoids the need for operator intervention. Nonetheless, both these solutions have further problems insofar as the process consumes a plate. This increases the cost of the test, which should be performed periodically to ensure that the power of the source is unchanged and that there have been no changes in the plate chemistry, due to ageing of the plate or the chemical used to develop the plate, for example.
In general, according to one aspect, the invention features a method for characterizing exposure levels in a platesetter. This method comprises exposing regions of a plate at different exposure levels by modulating the power of an exposure beam and then analyzing the plate after development to determine a desired exposure level for plates on the platesetter by reference to the regions receiving the different exposure levels.
The step of exposing the regions of the plate comprises exposing the plate in an area outside of a printing area. In this way, the exposure test can be performed as part of the normal plate printing process. The plate is not consumed. Moreover, the plate exposure setting can be constantly monitored throughout production, in one implementation.
In the preferred embodiment, the step of exposing regions of a plate comprises exposing the plate along a gripper edge, which is outside the printing area. This is typically an unused portion of the plate.
Further, the step of exposing regions of the plate comprises generating an exposure map. This map provides log changes in the exposure levels, by controlling a current to a laser, generating the exposure beam.
In other embodiments, where, for example, attenuators are located between the source and the media or plate, the exposure map is generated by controlling the attenuator. In other implementations, the step of generating the exposure map further comprises generating regions, including half-tone patterns, at different exposure levels, to more fully test the various exposure settings.
In general, according to another aspect, the invention features an exposure characterization system for a platesetter. This system comprises an exposure beam generator for generating a modulated exposure beam for exposing a plate. A controller then controls the exposure beam to expose regions of the plate at different exposure levels by controlling a power of the exposure beam.
In the preferred embodiment, this system comprises an exposure map storage for storing exposure levels of an exposure map, defining the different exposure levels.
In general, according to still another aspect, the invention features a method for controlling plate exposure in a plate production line. This comprises exposing the regions of a plate at different exposure levels in areas outside of the printing areas of the plate. The plates are then analyzed after the development to determine desired exposure levels for the plates, on the platesetter, by reference to the regions receiving the different exposure levels. This information is then fed back as changes to the exposure level for the printing areas of the plate.
Finally, according to still another aspect, the invention features a plate exposure level control system. This comprises an exposure beam generator for generating a modulated exposure beam for exposing the plates, and a controller for controlling the exposure beam generator to expose regions of the plates at different exposure levels by controlling a power of the exposure beam. A post-development analyzer is provided for analyzing the plates after development to determine a desired exposure level for the plates on the platesetter, by reference to the regions receiving the different exposure levels and feeding back changes in the exposure level for the printing areas of the plates in the platesetter.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.